Linear and branched chemoselective siloxane polymers and methods for use in analytical and purification applications

ABSTRACT

This invention relates generally to a new class of chemoselective polymer materials. In particular, the invention relates to linear and branched polysiloxane compounds for use in various analytical applications involving sorbent polymer materials, including chromatography, chemical trapping, analyte collection, and chemical sensor applications. These polymers have pendant and terminal aryl, alkyl, alkenyl, and alkynyl groups that are functionalized with halogen substituted alcohol or halogen substituted phenol groups, having the general structure:  
                 
 
     wherein:  
     n is an integer greater than  1;    
     wherein at least one of R 1  and R 2  includes an alkyl, alkenyl, alkynyl, or aryl group having at least one halogen substituted alcohol or halogen substituted phenol group attached thereto;  
     wherein any said R 1  and R 2  aryl groups are attached to said [Si—O—] n , the polymer backbone, directly or through a short hydrocarbon chain;  
     wherein any remaining R 1  or R 2  group is an alkyl, alkenyl, alkynyl, or aryl group having between one and sixteen carbons; and  
     Z is a polymer end group independently selected from the group consisting of saturated hydrocarbons, unsaturated hydrocarbons, alkyl silanes, aryl silanes, hydroxyl, hydride, alkoxides, halogen substituted alcohol, halogen substituted phenol, and combinations thereof.  
     These polymeric materials are primarily designed to sorb hydrogen bond basic analytes such as organophosphonate esters (nerve agents and precursors) and nitro-substituted compounds (explosives).

[0001] This application claims the benefit of a provisional application, U.S. Serial No.60/215,070 filed Jun. 30, 2000, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to a new class of chemoselective polymer materials. In particular, the invention relates to linear and branched polysiloxane compounds for use in various analytical applications involving sorbent polymer materials, including: chromatoghraphy, chemical trapping, and chemical sensor applications. These polymeric materials are primarily designed to sorb hydrogen bond basic analytes such as organophosphonate esters (nerve agents and precursors), and nitroaromatics (explosives).

[0004] 2. Description of the Related Art

[0005] The use of sorbent chemoselective polymers for chromatography, chemical trapping, and chemical sensor applications is well established for technologies such as gas liquid chromatography, solid phase microextraction (SPME), and surface acoustic wave (SAW) sensors respectively. In each application, the sorbent polymer is applied to a substrate as a thin film and analytes are sorbed to the polymer material. A typical configuration for a chemical sensor incorporates a thin layer of sorbent polymer deposited on a transducer that monitors changes in the physicochemical properties of the polymer film and translates these changes into an electrical signal that can be recorded. By careful design of the polymer, both sensitivity and selectivity of a chemical sensor can be enhanced with respect to specific classes or types of analytes. Typically, a chemoselective polymer is designed to contain functional groups or active sites that can interact preferentially with the target analyte through dipole-dipole, van der Waals, or hydrogen bonding forces. The interaction between a chemoselective polymer and the analyte can even be regarded as a “lock and key” type interaction if multiple active sites in the polymer are spatially controlled so that an analyte with multiple functional sites can simultaneously interact with the polymer active sites.

[0006] The ideal polymer film for extended chemical sensor applications should exhibit reversible binding of analyte, high selectivity and high sorptivity, long term stability; and, as a thin film, offer fast sorption and desorption properties. To achieve these characteristics a polymer must have physical properties that are amenable to rapid analyte sorption and desorption, suitable choice of functional groups, and a high density of functional groups to increase the sorptive properties for target analytes. Polymers with suitable analyte sorption characteristics can be obtained commercially for most analytes of interest with the exception of hydrogen bond acid polymers for sorption of hydrogen bond basic vapors. Of the few polymers that are commercially available (such as polyvinylalcohol, polyphenol, and fomblin zdol), either the physical properties are not ideal with glass transition temperatures above room temperature, the hydrogen bond acidity is relatively weak, or the density of functional groups is low.

[0007] Fluorinated polymers with hydroxyl groups as part of the polymer repeating unit and, in particular, polymers containing the hexafluoroisopropanol (HFIP) functional group are a well established class of hydrogen bond acid polymers. (See McGill, R. A.; Abraham, M. H.; Grate, J. W. CHEMTECH 1994,24 (9), 27; Ballantine, D. S.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986, 58,3058; Snow, A. W.; Sprague, L. G.; Soulen, R. L.; Grate, J. W.; Wohltjen, H. J. Appl. Pol. Sci., 1991, 43, 1659; Houser, E. J.; McGill, R. A.; Mlsna, T. E.; Nguyen, V. K.; Chung, R.; Mowery, R. L. Proc. SPIE, Detection and Remediation Technologies for Mines and Minelike Targets IV, Orlando, Fla., 1999, 3710, 394-401; Houser, E. J.; McGill, R. A.; Nguyen, V. K.; Chung, R.; Weir, D. W. Proc. SPIE, Detection and Remediation Technologies for Mines and Minelike Targets V, Orlando, Fla., 2000, 4038; Houser, E. J.; Mlsna, T. E.; Nguyen, V. K.; Chung, R.; Mowery, R. L.; McGill, R. A. Talanta, 2001, 54,469 ; Grate, J. W.; Patrash, S. J.; Kaganove, S. N.; Wise, B. M. Anal. Chem. 1999, 71,1033; all of which are incorporated herein by reference). The polymer fluoropolyol (FPOL) has become a standard material for many polymer based chemical sensor applications requiring hydrogen bond-acid polymers. (See: Ballantine, D. S.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986, 58, 3058; Snow, A. W.; Sprague, L. G.; Soulen, R. L. ; Grate, J. W.; Wohltjen, H. J. Appl. Pol. Sci., 1991, 43, 1659; all of which are incorporated herein by reference). Recently reported polymers such as BSP3, SXFA, and CS3P2 have yielded improvements in sensitivity and response time relative to FPOL. (See: Houser, E. J.; McGill, R. A.; Mlsna, T. E.; Nguyen, V. K.; Chung, R.; Mowery, R. L. Proc. SPIE, Detection and Remediation Technologies for Mines and Minelike Targets IV, Orlando, Fla., 1999,3710,394-401; Houser, E. J.; McGill, R. A.; Nguyen, V. K.; Chung, R.; Weir, D. W. Proc. SPIE, Detection and Remediation Technologies for Mines and Minelike Targets V, Orlando, Fla., 2000, 4038; all of which are incorporated herein by reference).

[0008] Determining and/or monitoring the presence of certain chemical species within a particular environment, e.g., pollutants, toxic substances and other predetermined compounds, is becoming of increasing importance with respect to such areas as defense, health, environmental protection, resource conservation, police and fire-fighting operations, and chemical manufacture. Devices for the molecular recognition of noxious species or other analytes typically include (1) a substrate and (2) a molecular recognition coating upon the substrate. These devices may be used, for example, as stand-alone chemical vapor sensing devices or as a detector for monitoring different gasses separated by gas chromatography. Molecular recognition devices are described in Grate et al., Sensors and Actuators B, 3, 85-111 (1991); Grate et al., Analytical Chemistry, Vol. 65, No. 14, Jul. 15, 1993; Grate et al., Analytical Chemistry, Vol. 65, No. 21, Nov. 15, 1993; and Handbook of Biosensor and Electronic Noses, ed. Kress-Rogers, CRC Press, 1996; all of which are incorporated herein by reference.

[0009] Frequently, the substrate is a piezoelectric material or an optical waveguide, which can detect small changes in the mass or refractive index, respectively. One illustrative example of a device that relies upon selective sorption for molecular recognition is known as a surface acoustic wave (SAW) sensor. SAW devices function by generating mechanical surface waves on a thin slab of a piezoelectric material, such as quartz, that oscillates at a characteristic resonant frequency when placed in a feedback circuit with a radio frequency amplifier. The oscillator frequency is measurably altered by small changes in mass and/or elastic modulus at the surface of the SAW device.

[0010] SAW devices can be adapted to a variety of gas and liquid phase analytical problems by designing or selecting specific coatings for particular applications. The use of chemoselective polymers for chemical sensor application is well established as a way to increase the sensitivity and selectivity of a chemical sensor with respect to specific classes or types of analytes. Typically, a chemoselective polymer is designed to contain functional groups that can interact preferentially with the target analyte through dipole-dipole, van der Waals, or hydrogen bonding forces. For example, strong hydrogen bond donating characteristics are important for the detection of species that are hydrogen bond acceptors, such as toxic organophosphorus compounds. Increasing the hydrogen bond acidity and the density of hydrogen bond acidic binding sites in the coating of a sensor results in an increase in selectivity and sensitivity of the sensor for hydrogen bond basic analytes.

[0011] Chemoselective films or coatings used with chemical sensors have been described by McGill et al. in Chemtech, Vol. 24, No. 9, 27-37 (1994), incorporated herein by reference. The materials used as the chemically active, selectively absorbent layer of a molecular recognition device have often been polymers, as described in Hansani in Polymer Films in Sensor Applications (Technomic, Lancaster, Pa. 1995). For example, Ting et al. investigated polystyrene substituted with hexafluoroisopropanol (HFIP) groups for its compatibility with other polymers in Journal of Polymer Science: Polymer Letters Edition, Vol.18,201-209 (1980). Later, Chang et al. and Barlow et al. investigated a similar material for its use as a sorbent for organophosphorus vapors, and examined its behavior on a bulk quartz crystal monitor device in Polymer Engineering and Science, Vol. 27, No. 10, 693-702 and 703-15 (1987). Snow et al. (NRL Letter Report, 6120-884A) and Sprague et al. (Proceedings of the 1987 U.S. Army Chemical Research Development and Engineering Center Scientific Conference on Chemical Defense Research, page 1241) reported making materials containing HFIP that were based on polystyrene and poly(isoprene) polymer backbones, where the HFIP provided strong hydrogen bond acidic properties. These materials were used as coatings on molecular recognition devices, such as SAW sensors, and showed high sensitivity for organophosphorus vapors. However, both the parent polymers and the HFIP-containing materials were glassy or crystalline at room temperature. Because vapor diffusion may be retarded in glassy or crystalline materials, the sensors produced were slow to respond and recover. Additional information is reported in Polym. Eng. Sci., 27, 693 and 703-715 (1987).

[0012] Grate et al. in Analytical Chemistry, Vol. 60, No. 9, 869-75 (1988), which is incorporated herein by reference, discloses a compound called “fluoropolyol” (FPOL), which is useful for detecting organophosphorus compounds. FPOL has the formula:

[0013] An HFIP-containing polymer based on a polysiloxane backbone was described and demonstrated to be a good hydrogen-bond acid by Abraham et al., “Hydrogen Bonding XXIX. The Characterisation of Fourteen Sorbent Coatings for Chemical Microsensors Using a New Salvation Equation”, J. Chem. Soc., Perkin Trans. 2, 369-78 (1995), herein incorporated by reference. The polysiloxane backbone provided a material with a Tg well below room temperature. However, physical properties were not quantified.

[0014] Grate, U.S. Pat. No. 5,756,631, incorporated herein by reference, discloses the use of HFIP-substituted siloxane polymers having the structure:

[0015] wherein R₂ has the formula —(CH₂)_(m−1)—CH═CH-CH₂—C(CF₃)₂—OH, n is an integer greater than 1, R₁ is a monovalent hydrocarbon radical, and m is 1 to 4.

[0016] Grate et al., U. S. Pat. No. 6,015,869, incorporated herein by reference, discloses a strongly hydrogen bonding acidic, sorbent oligomer or polymer having a glass-to-rubber transition temperature below 25° C. The polymer has (1) fluoroalkyl-substituted bisphenol segments containing interactive groups and (2) oligodimethylsiloxane segments. These siloxane polymers are said to provide improved coatings and vapor sorption compositions for chemical sensors that are sensitive, reversible and capable of selective absorptions for particular vapors, particularly the hydrogen bond accepting vapors, such as organophosphorus compounds.

[0017] The present invention discloses a newly discovered class of siloxane polymers that can be used to produce hydrogen bond acidic coatings for chemical sensor applications. Use of these siloxane polymers with highly functionalized units can result in significant selectivity and sensitivity improvements.

[0018] Further, the chemoselective siloxane polymer materials of the present invention exhibit, not only improved sensitivity to organophosphorus species, but also high selectivity and sensitivity toward nitroaromatic vapors, and are thus also useful for detecting the presence of explosives. Conventional explosives, such as trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5-trinitro-1,3,5,7-tetrazocine (HMX), may be contained in unexploded munitions, e.g., buried below the surface of the ground. Such munitions exude or leak vapors of the explosive. These vapors are typically concentrated in the surrounding soil and then migrate to the surface where they can be detected by the compounds, devices and methods disclosed by the present invention.

[0019] It is well known that dogs can be used to locate land mines demonstrating that the canine olfactory system is capable of detecting and identifying explosive related analyte signatures. In order to improve land mine detection capability, the use of sensors for the detection of chemical vapors associated with explosives is of great interest. Of particular interest in developing chemical sensors is the ability to detect unexploded ordnance, e.g., the polynitroaromatic compounds that are frequently present in the chemical signature of land mines.

SUMMARY OF THE INVENTION

[0020] This invention relates to the preparation of new linear and branched chemoselective polymeric materials for chemical sensor, chromatography, dosimeter, analyte collector, and air filtration applications.

[0021] According to a first aspect of the present invention, there is provided a siloxane polymer with pendant and terminal groups that are functionalized with halogen substituted alcohol or phenol groups having the general structure:

[0022] wherein n is an integer greater than 1;

[0023] wherein at least one of R¹ and R² includes an aryl or alkynyl group having at least one halogen substituted alcohol or halogen substituted phenol group attached thereto, or wherein both R¹ and R² are alkenyl groups having at least one halogen substituted alcohol group attached thereto;

[0024] wherein any said R¹ and R² aryl groups are attached to said [Si—O—]_(n), the polymer backbone, directly or through a short hydrocarbon chain;

[0025] wherein any remaining R¹ or R² group is an alkyl, alkenyl, alkynyl, or aryl group having between one and sixteen carbons; and

[0026] wherein Z is an end group independently selected from the group consisting of saturated hydrocarbons, unsaturated hydrocarbons, alkyl silanes, aryl silanes, hydroxyl, silicon hydride, alkoxides, halogen substituted alcohol, halogen substituted phenol, and combinations thereof.

[0027] According to a second aspect of the invention, there is provided a siloxane polymer with pendant and terminal aryl or allyl groups that are functionalized with halogen substituted alcohol or halogen substituted phenol groups that are primarily designed for the sorption of organophosphonate esters, nitroaromatics, and other hydrogen-bonding basic analytes.

[0028] According to a third aspect of the invention, there is provided a device for selective molecular recognition, the device comprising a sensing portion, wherein the sensing portion includes a substrate or multiple substrates having coated thereon a layer, the layer comprising any one of the polymeric compounds of the invention.

[0029] According to another aspect of the invention, there is provided a method of detecting a hydrogen bond basic analytes such as the organophosphonate esters or nitroaromatic compounds, comprising the steps of:

[0030] (a) contacting the molecules of such a analyte with the sensing portion of the device of the invention;

[0031] (b) collecting the molecules on the layer of the device, the molecules altering a specific physical property of the layer; and

[0032] (c) detecting the amount of change with respect to the physical property from before the contacting step (a) and after the collecting step (b).

[0033] According to a yet another aspect of the invention, there is provided a chemical vapor collector comprising an amount of any one of the polymeric compounds of the invention effective to collect hydrogen bond basic vapors or nitroaromatic compounds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The present invention relates to the preparation of new linear and branched chemoselective polymeric materials for chemical sensor, chromatography, analyte dosimeter, analyte collector, and air filtration applications. These polymers have pendant and terminal groups that are functionalized with halogen substituted alcohol or halogen substituted phenol groups. Pendant aryl groups can be attached to the siloxane polymer backbone either directly or through a hydrocarbon chain. These polymeric materials are primarily designed for the chemical detection of organophosphonate esters (nerve agents and precursors), and nitroaromatics (explosives) but may also have applications in detecting other hydrogen-bonding basic analytes.

[0035] In designing sorptive polymers for nitroaromatic analytes, hexafluoroisopropanol (HFIP) functionalized terminal alkene groups or aryl rings were chosen as the interactive portion of the polymer because of the high hydrogen-bonding acidity of these groups. Polynitroaromatic compounds possess multiple basic sites through the oxygen atoms of the nitro group and the hydrogen-bond acidity of the hexafluoroisopropanol group is complimentary to these basic sites. The hydrogen bond acidity of alcohols increases with the number of perfluoroalkyl groups bound to the carbinol group making the HFIP group an excellent hydrogen bond acid. In addition, the hydrogen bond basicity imparted by the oxygen atom of the hydroxyl group is substantially reduced thereby increasing the selectivity of the hydroxyl group for hydrogen bond basic analytes.

[0036] In addition to contributing to the hydrogen bond acidity of the hydroxyl group in the polymer, the fluorocarbon group also imparts substantial chemical stability to the polymer due to the inertness of the C—F bond. A further advantage is the steric bulk of the CF₃ groups and phenyl rings which hinders access to the polymer backbone thereby decreasing the effective hydrogen bond basicity of the polymer by limiting access to any electron rich atoms, such as oxygen, present in the polymer backbone. Use of the aromatic pendant groups provides two additional advantages in that they generally lead to more hydrogen bond acidic systems than comparable saturated hydrocarbons and they are better spatially oriented to interact with the electron rich oxygen atoms of the nitro groups on the nitroaromatic analytes.

[0037] The nitroaromatic analytes are dipolar and highly polarizable molecules that exhibit hydrogen-bond basic properties increasing with the number of nitro groups on the molecule. The hydrogen bond acidic polymers are designed to interact with the available electron density located on the oxygen atoms of the nitro groups of the polynitroaromatics. The hydrogen-bond basicities of some common nitroaromatics are 0.25 for 3-nitrotoluene, 0.47 for 2,4-dinitrotoluene and 0.61 for 2,4,6-trinitrotoluene demonstrating that the basicity of additional nitro groups is additive. These hydrogen bond basicities can be compared to those of hexane (0.0) and toluene (0.14). It should also be noted that the nitroaromatics are relatively large molecules and therefore also have significant van der Waals interactions with other materials.

[0038] In order to develop hydrogen bond acid polymers with improved physicochemical properties, preparation of new polymers with higher density of perfluoroalcohol functional groups and physical properties amenable to rapid vapor sorption/desorption kinetics have been targeted. Aryl rings were found to be preferable to the allyl group as a framework for fluoroalcohol functionalization because the hexafluoroisopropanol substituted allyl tends to exhibit a higher sorptivity for water molecules.

[0039] The compounds of the present invention can be synthesized by reacting hexafluoroacetone with the parent molecule, comprising a core polymer and a number of pendant unsaturated groups, taking advantage of the reactivity of perfluoroketones with terminally unsaturated groups, as described by Urry et al., J. Org. Chem., Vol. 33,2302-2310 (1968), incorporated herein by reference.

[0040] Once synthesized, these polymers can be coated to a controlled film thickness on a substrate, either alone or mixed with a solvent or similarly functionalized polymer. Useful substrates include planar chemical sensors, such as surface acoustic wave (SAW) substrates; optical fibers; and the interior surfaces of capillaries. The substrate chosen is based on the sensing mechanism being used.

[0041] The principle of operation of an acoustic wave device transducer involves the production of an acoustic wave that is generated on the surface or through the bulk of a substrate material and allowed to propagate. To generate the acoustic wave typically requires a piezoelectric material. Applying a time varying electric field to the piezoelectric material will cause a synchronous mechanical deformation of the substrate with a coincident generation of an acoustic wave in the material. The time varying electric field is generated in the surface by the action of the time varying electrical field applied through one or more electrodes that are connected to the piezoelectric material via one or more metal wire bonds and to an electrical circuit. Another electrode or electrodes receives the wave at a distance from the first electrode or electrodes. The second electrode or electrodes is also connected via metal wire bonds to the electrical circuit and the piezoelectric material. Such devices are operable in a frequency range of about 1 kilohertz to 10 gigahertz, preferably from about 0.2 megahertz to about 2 gigahertz and, more preferably, in the range of between about 200 to 1000 megahertz.

[0042] For piezoelectric sensors, piezoelectric substrates known in the art are useful in accordance with the invention, e.g., ST-cut quartz. In addition to quartz crystals, piezoelectric ceramics, such as those of the barium titanate and lead titanate zirconate families, are suitable substrates. These include LiNbO₃; BaTiO₃; 95 wt. % BaTiO₃/5% GaTiO₃; 80 wt. % BaTiO₃/12% PbTiO₃/8% CaTiO₃; PbNb₂O₆; Na_(0.5)K_(0.5)NbO₃; PbO_(0.94)Sr_(0.06)(Ti_(0.48)SrO_(0.52))O₃; and Pb_(0.94)(Ti_(0.48)Sr_(0.52))O₃. In some cases, the substrate may comprise a piezoelectric coating material, such as ZnO or AlN, applied to a non-piezoelectric material, such as a silicon or silicon carbide surface used in a micromachined device. The piezoelectric properties of these and other suitable materials are provided in CRC Handbook of Materials Science, Vol. III, Charles T. Lynch, CRC Press: Boca Raton, 198 (1975).

[0043] The sensing portion of an acoustic wave device of the invention is the area under the chemoselective layer, where the chemoselective layer covers the transducer. The area of the sensing portion of such a device can be on the order of about 0.0001-10 cm².

[0044] An optical waveguide chemical sensor consists of a light source, an optical waveguide, a chemoselective film or layer, and a detector to analyze the light after interacting with the layer. The waveguide is used to propagate light to a sensing portion of the device that contains the chemoselective layer. The light travels towards this coating and interacts with it. If the analyte being detected is present in the layer, the optical characteristics of the light may be altered, and the change is detected by some optically sensitive detector. In certain cases, the chemoselective layer may consist of a composite of polymer and one or more dyes.

[0045] An optical chemical sensor, commonly referred to as an optrode, includes a light source such as a semiconductor laser, light-emitting diode, or a halogen lamp; an optical waveguide such as a fiber optic or a planar waveguide substrate; a chemoselective layer deposited on the sensing portion of the optrode exposed to an analyte; and a detector to monitor the optical characteristics of the optrode. Sorption of the analyte to the chemoselective layer modifies the optical characteristics of the optrode, and this is detected as a change in refractive index or light intensity at one or more wavelengths of light. Thus, for optical sensors, optical fibers and optical wave guides are useful and are known in the art.

[0046] Fiber optic waveguides for sensor applications are commonly manufactured from silica glass or quartz as the core of the fiber. Surrounding this core is a cladding material that exhibits a lower refractive index than the cladding to achieve internal reflectance. The chemoselective layer is typically applied at the distal tip of the fiber optic or along the side of the fiber optic where a portion of the cladding material has been removed.

[0047] Planar waveguide optical sensors use a planar substrate device as a light guide. The use of a planar waveguide normally involves the use of evanescent wave techniques to take advantage of the large active surface area. Many of these sensors use the fluorescent properties of a chemoselective layer and are thus called Total Internal Reflection Fluorescence (TIRF) sensors.

[0048] Preferably, SAW devices are used as the substrate for the device of the invention. Particularly preferred SAW devices are 915 MHz two-port resonators made of ST-cut quartz with aluminum metallization and a thin silicon dioxide overcoat. SAW resonators and oscillator electronics to drive them are available from RF Monolithics and SAWTEK Inc.

[0049] Before application of a coating to form the sensor portion of the device of the invention, the substrate is cleaned. The cleaning procedure typically involves rinsing the device in an organic solvent and then subjecting it to plasma cleaning, as is well-known. Optionally, the substrate can be silanized with a material such as diphenyltetramethyldisilazane (DPTMS) by immersing the cleaned substrate surface in liquid DPTMS, placing the immersed surface into a partially evacuated chamber while heating the device to about 170° C. for about 12 hours. The silanized substrate is then removed and solvent cleaned with, for example, toluene, methanol, chloroform, or a physical or serial combination thereof, before applying the layer of the sensor portion of the device.

[0050] The method used for coating the compounds of the invention onto a substrate is not critical, and various coating methods known in the art may be used. Typically, the coating is applied to the substrate in solution, either by dipping, spraying or painting, preferably by an airbrush or spin coating process. The concentration of the compound of the invention in the coating solution should be sufficient to provide the viscosity most appropriate for the selected method of coating, and may easily be determined empirically. The solvent used, although not critical, should be sufficiently volatile as to facilitate quick and easy removal, but not so volatile as the complicate the handling of the coating solution prior to being deposited on the substrate. Examples of useful solvents include, for example, hexane, chloroform, methanol, toluene, tetrahydrofuran, and water. J. W. Grate and R. A. McGill in Analytical Chemistry, Vol. 67, No. 21, 4015-19 (1995), the subject of which is incorporated herein by reference, describe making chemical acoustic wave detectors by applying a thin film to a surface acoustic wave device. The thickness of the chemoselective layer preferably does not exceed that which would reduce the frequency of a chemical sensor operating at 250 megahertz by about 250 kilohertz and, typically, is in the range of about 0.5 nm to 10 microns, preferably in the range of 1 to 300 nm.

[0051] The coating may comprise a single layer or multiple layers. With multiple layers, a layer containing the compound of the invention may be combined with at least one other layer that provides pores suitable for physically eliminating some chemical species of large size that are not to be monitored.

[0052] The process of sorption plays a key role in the performance of chemical sensors for gas phase analysis. For example, microsensors, which consist of a physical transducer and a selective sorbent layer, sense changes in the physical properties, such as mass, in the sorbent layer on the surface of the transducer, due to the sorption of analyte molecules from the gas phase into the sorbent layer. Coating material properties that are known to elicit a detectable SAW sensor response are mass (i.e., as determined by the thickness and density of the coating), elasticity, viscoelasticity, conductivity, and dielectric constant. Changes in these properties can also result in changes in the attenuation (i.e., loss of acoustic power) of the wave. In some situations, monitoring attenuation may be preferable to monitoring velocity. Alternatively, there are some situations where simultaneously monitoring both velocity and attenuation can be useful. In any event, it is the modification of the sensed properties of the sorbent layer, as a result of analyte sorption, that results in the measurable response when analyte molecules are present in the gas or liquid phases being monitored. SAW devices coated with compounds of the invention are capable of detecting mass changes as low as about 100 pg/cm². Further, vapor diffusion is rapid providing fast detection capability in a sub second time frame.

[0053] Sensor selectivity, the ability to detect a chemical species in an environment containing other chemical species, is generally determined by the ability of the coated layer to specifically sorb the species to be detected to the exclusion of others. For most coatings, selectivity is obtained based on providing stronger chemical interactions between the coated layer and the target species than occurs between the layer and species that are not to be detected. The method of selectively detecting the presence of a chemical entity within an environment comprises (a) placing the sensing portion of the device of the invention in the environment and (b) detecting changes in the coated layer of the sensing portion of the device. The environment may be gaseous or liquid.

[0054] More than one device may be provided. For example, a plurality of sensor portions could be used in a sensor array with, e.g., associated control devices and software, in a manner similar to conventional procedures employed with sensor arrays.

[0055] After an initial sensing has taken place, the coated sensor layer can be purged or cleaned by a second stream, allowing sensing of a new third stream to take place. For example, air, water- or acid-base solutions could be used as purging or cleaning solutions, depending on the species being detected and the nature of the layer.

[0056] In the devices and methods of the invention, the compounds are excellent sorbents for both hydrogen bond basic vapors, such as organophosphorus compounds, and also for nitroaromatic materials, such as explosives. It is expected that the chemical sensor systems of the present invention could weigh between 1-32 ounces and could, therefore, be easily mounted on a remote or robotic vehicle for automatically detecting buried explosives or munitions. Alternatively, such a device would also be useful for remotely detecting chemical agents or explosives secreted upon a person intending the destruction of private property and/or personnel, such as, for example, at crowded public places like airports or arenas where terrorist activities may be suspected.

[0057] If desired, it is possible to increase the concentration of explosive vapors contained in the area being monitored, i.e., speed up their release from buried or otherwise hidden munitions or explosives, by irradiating the area with electromagnetic radiation. Increasing the concentration of vapor in the soil or other environment surrounding a munition will produce a stronger signal following the reaction with sensor portion of the device of the present invention.

[0058] The chemoselective, siloxane polymers of the invention exhibit high selectivity and sensitivity toward nitroaromatic vapor, due at least in part to the sensitivity and selectivity of the multiple 1,1,1,3,3,3-hexafluoroisopropanol groups that are present. The presence of these functional groups is also directly responsibility for the sensitivity of these materials to hydrogen bond basic vapors. The functionalized polysiloxane compounds of the invention also have the advantage of high-yield preparation methods, ready purification, in addition to having an increased density of functional groups, as compared with polymeric coatings.

[0059] Moreover, the flexibility in the synthesis of these materials allows one to tailor a wide variety of related chemoselective compounds.

EXAMPLES Example 1 Synthesis Procedures

[0060] Polybis(phenpropyl)siloxane: To a 250 mL Schlenk flask containing a solution of 25% dichlorosilane in xylenes (5.0 g, 5.8 mL, 12.5 mmol) was added allylbenzene (4.43 g, 5.0 mL, 37.5 mmol) and treated with a catalytic amount (˜0.5 mg) of hexachloroplatinic acid. The resulting solution was stirred for 14 hours at room temperature then heated to reflux for 6 hours. After cooling to room temperature, the solution was cooled in an ice bath and treated with a 50/50 mixture (200 mL) of diethyl ether and water. The solution was then treated with chlorotrimethylsilane (1.0 mL, 9.2 mmol) and stirred for 16 hours at room temperature. Aqueous work-up, filtration and removal of solvent gave a colorless viscous polymer. Yield: 98%. FTIR (NaCl, cm⁻¹): 3078, 3058, 3034, 2927, 2852, 2795, 1603,1495, 1087, 1028.

[0061] Polymethyl(phenpropyl)siloxane: To a 250 mL Schlenk flask containing a solution of polymethylhydrosiloxane, trimethylsilyl terminated (3.0) in tetrahydrofuran (30 mL) was added excess allylbenzene (15.0 g, 16.8 mL, 12.7 mmol). The resulting solution was treated with a catalytic amount (˜0.5 mg) of hexachloroplatinic acid and stirred at room temperature for 14 hours. The solution was then heated to reflux for 4 hours and cooled to room temperature. Aqueous work-up and removal of solvent gave a colorless viscous polymer. Yield: 97%. FTIR (NaCl, cm⁻¹): 3083, 3057,3046,2927,2852, 1621, 1512, 1461, 1268, 1085, 1031.

[0062] Polymethyl(2-naphthylethyl)siloxane: To a 250 mL Schlenk flask containing a solution of polymethylhydrosiloxane, trimethylsilyl terminated (0.5) in tetrahydrofuran (30 mL) was added excess 2-vinylnaphthalene (1.5 g, 9.7 mmol). The resulting solution was treated with a catalytic amount (˜0.5 mg) of hexachloroplatinic acid and stirred at room temperature for 14 hours. The solution was then heated to reflux for 4 hours and cooled to room temperature. Aqueous work-up and removal of solvent gave a brown viscous polymer. Yield: 97%. FTIR (NaCl, cm⁻¹): 3084, 3061, 3025, 3001, 2924, 2855, 2795, 1603, 1495, 1235, 1093, 1030.

Functionalization of Polymers

[0063] General Procedure for the Functionalization of Polymers with Hexafluoroacetone (HFA): A portion (2-10 g) of the polymer was intimately mixed with a catalytic amount of aluminum chloride (approx. 50 mg/g of polymer) and placed into a mild steel cylinder with a stir bar and the cylinder evacuated. Hexafluoroacetone (0.5-2.0 g) was vacuum transferred into the steel cylinder and the resulting mixture heated to 80° C. for 48 hours. Note: hexafluoroacetone is highly toxic and is dangerous under pressure. After cooling to room temperature, the volatiles were removed and the resulting polymer purified by extraction into CHCl₃ and washed with water. The CHCl₃ polymer solution was filtered and the solvent removed in vacuo leaving a viscous, pale brown polymer. The presence of the —(CF₃)₂COH group is verified by the presence of an O—H stretching absorptions in the 3500-3550 cm⁻¹ region of the FTIR spectrum of the functionalized polymer.

[0064] Reaction of polybis(phenpropyl)siloxane with hexafluoroacetone: Reaction of 3.0 g of polybis(phenpropyl)siloxane with hexafluoroacetone was performed according to the procedure above. Yield: 96%. FTIR (NaCl, cm⁻¹): 3589, 3508 (ν_(O—H)).

[0065] Reaction of polymethyl(phenpropyl)siloxane with hexafluoroacetone: Reaction of 5.0 g of polymethyl(phenpropyl)siloxane with hexafluoroacetone was performed according to the procedure above. Yield: 96%. FTIR (NaCl, cm⁻¹): 3590, 3501 (ν_(O—H)).

Example 1 Synthesis of bis(1-(4-hydroxy, 4-trifluoromethyl, 5,5,5-trifluoro)pentene), polysiloxane

[0066] In a 500 mL Schlenk flask, 10 grams of the precursor (CH₂—CHCH₂)₂SiCl₂ is added to 100 grams of dry ether at 0° C. Excess water (5 grams) is slowly added to the dry chlorosilane/ether mixture over 5 hours. The mixture is then stirred for 18 hours at room temperature. The resulting polymer, approximately 6.0 grams of [(CH₂═CHCH₂)₂SiO]n, where n is an integer greater than 1, is extracted with diethyl ether. The ether is removed by evaporation and the polymer heated at 70° C. for 18 hours under vacuum. The polymer is then reacted with hexafluoroacetone. In a stainless steal reaction chamber, 5.22 g of the polymer is dissolved in 40 grams of CHCl₃. The mixture is cooled to −196° C. and evacuated. Then 30 g of hexafluoroacetone is condensed into the reaction vessel. The mixture is then heated to 100° C. for 18 hours. Excess hexafluoroacetone is recovered and the polymer extracted with CHCl₃ The solvent is then removed and 11.95 g of functionalized polymer is collected.

[0067] Characterization—All regions of the ¹⁹F NMR are empty except for the narrow region corresponding to CF₃ groups. In this region, −75 to −81 (ref CF₃Cl) resonance peaks corresponding to the trifluoromethyl groups can be found. IR spectra contain bands characteristic of the fluoroalcohol siloxane polymer. The strong C—F region from 1000 to 1300 cm⁻¹, the sharp signal near 1615 cm⁻¹ associated with the C═C stretch, and the free hydroxyl at 3600 cm⁻¹ and the hydrogen-bonded hydroxyl at 3400-3500 cm⁻¹.

[0068] Elemental Anal: Found. C, 31.30; H, 2.03; Calcd for SiO₃C₁₂H₁₀F₁₂: C, 31.45; H 2.20. (SiO)(CH^(a)═CH^(b)CH₂ ^(c)C(CF₃ ^(m))₂OH^(d))₂ ¹H-NMR (CDCl3) a 6.75 (1H, s); b 5.78 (1H, s);c 2.73 (2H, s); d 2.92 (1H, s). ¹⁹F NMR (CFCl₃) m -77.3 (s). IR (neat) 3600, 3527, 3023, 2967, 1607, 1440, 1386, 1329, 1148, 1014, 984, 928, 840, 765, 705, 636, 574, 537 cm⁻¹.

Synthesis of Crossed Linked Polymers

[0069] The linear hexafluoroisopropanol functionalized polymers, similar to the material described in Example 1, are low viscosity liquids. The molecular weight and viscosity of these materials are increased by crosslinking. This produces a series of polymers from low viscosity liquids to solids without compromising the degree of functionality. 5.0 grams of the precursor CH₃(CH₂═CHCH₂)SiCl₂ and 5.7 grams CH₂═CHCH₂SiCl₃ of is added to 100 grams of dry ether at 0° C. Excess water (5 grams) is slowly added to the dry chlorosilane/ether mixture over 5 hours. The mixture is then stirred for 18 hours at room temperature. The resulting polymer, approximately 5.2 grams of [CH₃(CH₂═CHCH₂)SiO]n[(CH₂═CHCH₂)SiO₂]m, where the ration of n:m is approximately 1:1, is extracted with ether. The ether is removed by roto evaporation and the polymer cured at 70° C. for 18 hours under vacuum. ¹H NMR integration determines the degree of crosslinking from n=0 to m=0. Functionalization with hexafluoroacetone proceeds similarly to the previously described linear examples.

Synthesis of Hydrogenated Polymers

[0070] The hydrogenation of the linear and cross linked polymers is carried out using a palladium catalyst supported on charcoal. The catalyst, polymer and chloroform are placed in a stainless steel reaction vessel with a glass insert. The bomb is pressurized with hydrogen to 60 psi and then heated to 120° C. for 66 hours. The reaction mixture is filtered and the polymer is extracted with chloroform. Infrared spectroscopy confirms complete saturation of the C═C bonds in the polymer products.

Example 2 Applying a Thin Film to a SAW Device

[0071] SAW devices are cleaned in a Harrick plasma cleaner prior to polymer film application. Aerosol spray-coated films of the present invention in solvent are applied to a SAW device using an airbrush supplied with compressed dry nitrogen. The frequency change of the SAW device operating in an oscillator circuit is monitored during deposition, using the change in frequency as a measure of the amount of material applied. After application, the films are annealed at 50° C. overnight in an oven. Spray-coated films are examined by optical microscopy with a Nikon microscope using reflected light Nomarski differential interference contrast.

Example 3 Detection of Basic Vapors with a Compound-Coated SAW Device

[0072] The polymers of the present invention are applied to SAW devices and tested against organic vapors at various concentrations. Upon exposure to a vapor, the coated acoustic wave devices undergo a shift in frequency that is proportional to the amount of vapor sorbed by the compound. Times to steady state response, corresponding to equilibrium partitioning of the vapor into the compound layer, are typically under 15 seconds using a vapor delivery system. From frequency shift data for a vapor at multiple concentrations, calibration curves are constructed. The calibration curves are nonlinear, which is consistent with hydrogen bonding interactions at a finite number of sites in the polymers of the present invention.

[0073] Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed:
 1. A siloxane polymer having the general structure:

wherein: n is an integer greater than 1; wherein Z is an end group independently selected from the group consisting of saturated hydrocarbons, unsaturated hydrocarbons, organosilanes, hydroxyl, hydride, alkoxides, halogen substituted alcohols, halogen substituted phenols, and combinations thereof; wherein at least one of R¹ and R² includes an aryl group having at least one halogen substituted alcohol or halogen substituted phenol attached thereto; wherein any said R¹ and R² aryl groups are attached to said [Si—O—]_(n) directly or through a short hydrocarbon chain having 1 to 10 carbons; and wherein any remaining R¹ or R² group is an alkyl, alkenyl, alkynyl, or aryl group having between 1 and 16 carbons.
 2. The siloxane polymer of claim 1, wherein said R¹ is a methyl group.
 3. The siloxane polymer of claim 2, wherein said aryl group of said R² is a benzene ring having two said halogen substituted alcohol groups attached thereto.
 4. The siloxane polymer of claim 3, wherein said halogen substituted alcohol groups are —C(CF₃)₂—OH groups.
 5. The siloxane polymer of claim 2, wherein said R² contains said short hydrocarbon chain terminating in said aryl group, and said aryl group is a benzene ring having two said halogen substituted alcohol groups attached thereto.
 6. The siloxane polymer of claim 5, wherein said short hydrocarbon chain has 3 carbon atoms.
 7. The siloxane polymer of claim 6, wherein said 3 carbon short hydrocarbon chain is saturated.
 8. The siloxane polymer of claim 5, wherein said halogen substituted alcohol groups are —C(CF₃)₂—OH groups.
 9. The siloxane polymer of claim 7, wherein said halogen substituted alcohol groups are —C(CF₃)₂—OH groups.
 10. The siloxane polymer of claim 1, wherein each said R¹ and R² contain a said short hydrocarbon chain terminating in said aryl group, and said aryl group is a benzene ring having two said halogen substituted alcohol groups attached thereto.
 11. The siloxane polymer of claim 10, wherein said short hydrocarbon chains have 3 carbon atoms.
 12. The siloxane polymer of claim 10, wherein said halogen substituted alcohol groups are —C(CF₃)₂—OH groups.
 13. The siloxane polymer of claim 11, wherein said halogen substituted alcohol groups are —C(CF₃)₂—OH groups.
 14. The siloxane polymer of claim 1, wherein said aryl groups of R¹ and R² are each a benzene ring having two said halogen substituted alcohols attached.
 15. The siloxane polymer of claim 14, wherein said halogen substituted alcohol groups are —C(CF₃)₂—OH groups.
 16. The siloxane polymer of claim 1, wherein said remaining R¹ or R² group is an allyl group having one said halogen substituted alcohol attached thereto.
 17. The siloxane polymer of claim 1, wherein said remaining R¹ or R² group is an aryl group having one said halogen substituted alcohol group attached thereto.
 18. A siloxane polymer having the general structure:

wherein n is an integer greater than 1; wherein R¹ and R² are pendant groups having between 1 and 10 carbons each and are independently selected from the group consisting of alkyl, alkenyl, and alkynyl groups; and wherein each said pendant group has at least one halogen substituted alcohol or halogen substituted phenol group attached thereto.
 19. The siloxane polymer of claim 18, wherein said pendant groups are alkenyl groups consisting of an allyl group.
 20. The siloxane polymer of claim 19, wherein said allyl group terminates in one said halogen substituted alcohol or halogen substituted phenol group.
 21. A method of preparing a functionalized siloxane polymer, comprising the steps of: (a) selecting a core siloxane polymer having the structure:

wherein n is an integer greater than 1; wherein m is an integer greater than or equal to 0; wherein Z is an end group independently selected from the group consisting of saturated hydrocarbons, unsaturated hydrocarbons, alkyl silanes, aryl silanes, hydride, alkoxides, hydroxyl, and combinations thereof; and wherein R⁴, R⁵, and R⁶ are pendant groups independently selected from hydrocarbons containing 1 to 12 carbons; (b) selecting an unsaturated short chain hydrocarbon having between 1 to 10 carbons and having an aryl group attached thereto; (c) reacting said core siloxane polymer with said unsaturated short chain hydrocarbon in a solution of an hydrosilation catalyst, thereby forming a siloxane intermediate polymer having the structure:

wherein X is a hydrocarbon chain having an aryl group attached thereto; and (d) reacting said siloxane intermediate polymer with hexafluoroacetone or a halogen substituted alcohol or phenol source, thereby forming said functionalized siloxane polymer.
 22. The method as recited in claim 21, wherein said hydrosilation catalyst is hexachloroplatinic acid.
 23. The method as recited in claim 21, wherein said unsaturated short chain hydrocarbon has 3 carbons.
 24. The method as recited in claim 21, wherein in said substituted siloxane polymer, said R⁴ is a methyl group, said Z is trimethylsilyl, said m equals 0, and said aryl group has one or two said halogen substituted alcohol or phenol groups attached thereto.
 25. The substituted siloxane polymer of claim 24, wherein said two halogen substituted alcohol or phenol groups are —C(CF₃)₂—OH groups.
 26. The method as recited in claim 21, wherein said reaction of said siloxane intermediate polymer is conducted in a solution containing AlCl₃.
 27. The method as recited in claim 21, wherein said siloxane intermediate polymer is:


28. The method as recited in claim 21, wherein said functionalized siloxane polymer is:


29. A method of preparing a functionalized siloxane polymer, comprising the steps of: (a) selecting an unsaturated short chain hydrocarbon having between 1 and 10 carbons and an aryl group attached thereto; (b) reacting said unsaturated short chain hydrocarbon with H₂SiCl₂ or (CH₃)₃CSiHCl₂ in a solution containing a hydrosilation catalyst, thereby forming a dichlorosilyl intermediate; (c) performing a hydrolysis reaction with said dichlorosilyl intermediate in a solution containing (CH₃)₃SiCl, thereby forming a siloxane polymer intermediate; and (d) reacting said siloxane polymer intermediate with hexafluoroacetone or a halogen substituted alcohol or phenol source, thereby forming said functionalized siloxane polymer.
 30. The method as recited in claim 29, wherein said hydrosilation catalyst is hexachloroplatinic acid.
 31. The method as recited in claim 29, wherein said short chain hydrocarbon has 3 carbons.
 32. The method as recited in claim 29, wherein said substituted siloxane polymer contains two said aryl groups, each being a benzene ring having two said halogen substituted alcohol or phenol groups attached thereto.
 33. The substituted siloxane polymer of claim 32, wherein said substituted alcohol or phenol groups are —C(CF₃)₂—OH groups.
 34. The method as recited in claim 29, wherein said siloxane polymer intermediate is:

wherein n is an integer greater than 1;
 35. The method as recited in claim 29, wherein said functionalized siloxane polymer is:

wherein n is an integer greater than 1;
 36. The method as recited in claim 29, wherein said aryl group attached to said unsaturated short chain hydrocarbon is a naphthyl group.
 37. The method as recited in claim 36, wherein said unsaturated short chain hydrocarbon having between 1 and 10 carbons and an aryl group attached thereto is 2-vinyl naphthalene.
 38. The method as recited in claim 29, wherein said dichlorosilyl intermediate is:

wherein: p is a positive integer equal to or greater than 0;
 39. The method as recited in claim 29, wherein said bis(arylalkyl) siloxane polymer is:

wherein: p is a positive integer equal to or greater than 0;
 40. The method as recited in claim 29, wherein said functionalized siloxane polymer is:

wherein n is an integer greater than 1;
 41. A device for selective molecular detection, the device comprising a sensing portion, wherein said sensing portion includes a substrate having coated thereon a layer, said layer comprising the siloxane polymer of claim
 1. 42. A device for selective molecular detection, the device comprising a sensing portion, wherein said sensing portion includes a substrate having coated thereon a layer, said layer comprising the siloxane polymer of claim
 18. 43. The device of claim 41, wherein said substrate is a surface acoustic wave (SAW) substrate.
 44. The device of claim 42, wherein said substrate is a surface acoustic wave (SAW) substrate.
 45. A method of detecting the molecules of a hydrogen bond basic analyte, comprising the steps of: (a) contacting the molecules of said analyte with a device comprising a sensing portion, wherein said sensing portion includes a substrate having coated thereon a layer, said layer comprising the material of claim 1; (b) collecting said molecules in said layer, wherein said molecules alter a specific physical property of said layer; and (c) detecting the amount of change in the physical property from before said contacting step (a) and after said collecting step (b).
 46. A method of detecting the molecules of a hydrogen bond basic analyte, comprising the steps of: (a) contacting the molecules of said analyte with a device comprising a sensing portion, wherein said sensing portion includes a substrate having coated thereon a layer, said layer comprising the material of claim 18; (b) collecting said molecules in said layer, wherein said molecules alter a specific physical property of said layer; and (c) detecting the amount of change in the physical property from before said contacting step (a) and after said collecting step (b).
 47. The method of claim 45, wherein said substrate is a surface acoustic wave (SAW) substrate.
 48. The method of claim 46, wherein said substrate is a surface acoustic wave (SAW) substrate.
 49. A collection device for selective molecular sorption for molecules of a hydrogen bond basic analyte, wherein said device comprises the material of claim
 1. 50. A collection device for selective molecular sorption for molecules of a hydrogen bond basic analyte, wherein said device comprises the material of claim
 18. 51. A method of preparing a substituted crosslinked siloxane polymer, comprising the steps of: (a) performing a hydrolysis reaction of a mixture of dichloroorganosilanes and trichloroorganosilanes, thereby forming a crosslinked siloxane polymer having the structure:

wherein m is an integer greater than or equal to 0; wherein n is an integer greater than 1; wherein Z is an end group independently selected from the group consisting of saturated hydrocarbons, unsaturated hydrocarbons, alkyl silanes, aryl silanes, silicon hydride, alkoxides, 1,1,1,3,3,3-hexafluoroisopropanol, 2,3,5,6-tetrafluorophenol, hydroxyl, and combinations thereof; and wherein R⁴, R⁵, and R⁶ are pendant groups independently selected from hydrocarbons containing 1 to 12 carbons; (b) reacting said crosslinked siloxane polymer with hexafluoroacetone to form said substituted crosslinked siloxane polymer. 